This invention relates generally to method and apparatus for reducing or eliminating time-varying currents and voltages which may be present in an electrochemical cell or battery while it is in the process of being charged or is supplying power to an active load. More particularly, it relates to the problem of using a time-varying signal to measure a cell/battery's incremental parameters--such as its incremental conductance, resistance, or impedance--while the cell/battery is "on line" and therefore already conducting a time-varying current. By employing the present invention, such incremental parameters can be accurately measured during periods of normal use--without requiring that the cell/battery be removed from service. This invention therefore significantly increases the utility of dynamic conductance battery testing apparatus of the type disclosed previously in U.S. Pat. No. 5,140,269 issued to Keith S. Champlin. Additional applications of the present invention include eliminating ripple and crosstalk in batteries that are supplying voltage-sensitive equipment and preventing plate deterioration caused by ac currents flowing through a cell/battery.
Stationary lead-acid batteries are employed in many applications requiring energy to be delivered continuously over relatively long periods of time. Such batteries, comprised of banks of series-connected two-volt cells, are found at electric generating plants, substations, telephone central offices, railroad signal sites, airport control towers, and countless other critical installations to provide secondary emergency power for use in the event of failure of a primary energy source. Applications requiring relatively long-term reliance on such secondary batteries include emergency lighting for hospitals and industrial plants, and uninterruptible energy supplies for critical communications equipment and computers. Individual cells of stationary batteries are often separate entities with accessible terminals. Such cells may be physically quite large and will sometimes weigh hundreds of pounds.
The traditional method for testing stationary batteries, or their individual cells, is the timed-discharge test. This well-established procedure is fully described in Section 6 of ANSI/IEEE Standard 450-1987. Under this procedure, one removes the battery from service and discharges it at a constant current while simultaneously monitoring the terminal voltage of the battery and/or its individual cells. The cell/battery capacity is then calculated from the length of time that the discharge can be sustained before the appropriate voltage drops below a particular threshold value.
Although the timed-discharge test has seen widespread use, it possesses serious disadvantages. These include:
1. The test requires that the battery be removed from service for a considerable length of time (usually 8 or 10 hours).
2. Currents drawn may be relatively large and can thus require apparatus that is heavy and cumbersome.
3. After being tested, the battery must be recharged before it can be returned to service. This requires additional time.
4. Only a fixed number of charge-discharge cycles can be provided by a given battery. As a result, each timed-discharge test removes potential service capability.
Because of these disadvantages, measurements of small-signal incremental parameters such as resistance, impedance, and conductance, have been proposed as alternatives to the timed-discharge test. The motivation for this effort dates from the pioneering work of DeBardelaben (S. DeBardelaben, "Determining the End of Battery Life", INTELEC 86, Toronto, Canada, pp. 365-368). His paper disclosed a strong inverse correlation between a cell's capacity and either the magnitude of the cell impedance or its resistive real part. Additional studies by Vaccaro and Casson (F. J. Vaccaro and P. Casson, "Internal Resistance: Harbinger of Capacity Loss in Starved Electrolyte Sealed Lead Acid Batteries" INTELEC 87, Stockholm, Sweden pp 128-131) showed that increased impedance and resistance were also good indicators of "dryout" of sealed-lead acid stationary batteries.
In U.S. Pat. No. 5,140,269 referenced above, the present inventor disclosed apparatus for measuring incremental conductance of a cell/battery and showed that this quantity is related linearly to the capacity of the cell/battery. This assertion has been corroborated by many field measurements such as those described in a recent paper by Feder, et al. (D. O. Feder, et al. "Field and Laboratory Studies to Assess the State of Health of Valve-Regulated Lead Acid Batteries: Part I Conductance/Capacity Correlation Studies", INTELEC 92, Washington, D.C., pp. 218-233). Results of both timed-discharge tests and conductance measurements on approximately 500 cells are presented therein. These results indeed reveal a very linear relationship between battery/cell capacity and incremental conductance and display a high degree of correlation (R.sup.2 of 0.80 to 0.98).
In order to measure incremental resistance or impedance, one passes a time-varying current through a cell/battery and observes the appropriate component of the resulting time-varying voltage developed across it. Incremental conductance is measured in the opposite manner. One places a time-varying voltage across a cell/battery and observes the appropriate component of time-varying current passing through the cell/battery. In either type of measurement, a problem arises if there are already time-varying currents and voltages present. Such signals, when present during measurement, can degrade accuracy and may introduce serious error. Unfortunately, spurious time-varying signals are common occurrences for cells/batteries undergoing either "float" or high-rate charging, or supplying power to an "active" load. Under such circumstances, time-varying battery currents frequently result from imperfect filtering of the battery charger's rectifier or from fluctuations in the current drawn by the load.
One approach to solving problems introduced by charger/load-related signals is to simply remove the battery from service during measurement of its incremental parameters. This has, in fact, been done many times with satisfactory results. Many cases arise, however, in which it is not desirable or even feasible to take the cell/battery "off line."
A second approach is described in the prior art and is the approach followed by DeBardelaben in the reference cited above, as well as by Burkum and Gabriel in U.S. Pat. No. 4,697,134. Their approach is to choose the measurement frequency to be different from any frequencies that are otherwise present in the charger/load circuit and to then use filters to separate the measuring signal from the spurious signals. This solution to the problem is likewise not entirely satisfactory since it assumes prior knowledge of the spurious signal frequencies and requires that the measurement frequency be dictated by the characteristics of the charger/load circuit rather than by requirements of the cell/battery itself.